Cosette (French pronunciation: [kɔzɛt]) is a fictional character in the 1862 novel Les Misérables by Victor Hugo and in the many adaptations of the story for stage, film, and television. Her birth name, Euphrasie, is only mentioned briefly. As the orphaned child of an unmarried mother deserted by her father, Hugo never gives her a surname. In the course of the novel, she is mistakenly identified as Ursule, Lark, or Mademoiselle Lanoire.

She is the daughter of Fantine, a working woman who leaves her to be looked after by the Thénardiers, who exploit and victimise her. Rescued by Jean Valjean, who raises Cosette as if she were his own, she grows up in a convent school. She falls in love with Marius Pontmercy, a young lawyer. Valjean's struggle to protect her while disguising his past drives much of the plot until he recognizes "that this child had a right to know life before renouncing it"—and he must allow her romantic attachment to Marius to blossom.

Euphrasie (a name from Greek Ευφράσια, 'good cheer, joy'), nicknamed Cosette (lit. 'little thing') by her mother, is the illegitimate daughter of Fantine and Félix Tholomyès, a rich student. She is born in Paris c. 1816. Tholomyès abandons Fantine, who leaves three-year-old Cosette with the Thénardiers at their inn in Montfermeil, paying them to care for her child while she works in the city of Montreuil-sur-Mer. Unbeknownst to Fantine, the Thénardiers abuse Cosette while she is under their care. They beat and starve her and force her to perform heavy labor in the inn. Under the Thénardiers' care she is described as "thin and pale", wears rags for clothing, and has chilblains on her hands as well as bruised and reddened skin. She is forced to go barefoot in winter. The narrator also says that "fear was spread all over her".

While Fantine is in the hospital, the mayor of Montreuil-sur-Mer—Jean Valjean masquerading as "Madeleine" to avoid revealing that he is a paroled convict—vows to retrieve Cosette for her. Although Fantine dies before seeing her daughter, Valjean becomes determined to look after the young girl.

When he arrives in Montfermeil on Christmas Eve, Valjean finds Cosette fetching a pail of water and accompanies her back to the inn, where he witnesses her mistreatment by the Thénardiers as well as the unkindnesses of their daughters Éponine and Azelma. Valjean leaves the inn and returns with an expensive new doll which he offers to Cosette. At first reluctant, Cosette joyfully accepts it, which infuriates Thénardier; Éponine and Azelma become jealous.

The next morning, Christmas Day, Valjean informs the Thénardiers that he has come to take Cosette with him. Madame Thénardier immediately agrees but Thénardier pretends affection for Cosette and acts reluctant. Valjean pays 1500 francs to settle Fantine's debts and leaves with Cosette. Thénardier tries to swindle more money out of Valjean by saying that he has changed his mind and wants Cosette back, informing Valjean that Cosette's mother had entrusted her to his care and he cannot release her without a note from her mother. Valjean, agreeing with him, hands him a letter signed by Fantine. Thénardier then attempts to order Valjean to return Cosette or to pay a thousand crowns, but Valjean ignores him and leaves with Cosette.

Valjean takes Cosette to Paris, where they lodge at Gorbeau House; Valjean begins her education. When Inspector Javert discovers Valjean's whereabouts, he and Cosette must flee. Valjean climbs over a wall and they find themselves in a garden attached to a convent. The gardener Fauchelevent recognizes Valjean as the man who rescued him years earlier and agrees to shelter them. Valjean poses as Fauchelevent's brother and says he is Cosette's grandfather. They live peacefully in the convent for many years as Valjean works with Fauchelevent and Cosette attends the convent school. Over time, she appears to have no recollection of her childhood before arriving at the convent. As Cosette matures, beautiful and healthy, with chestnut hair, beautiful eyes, rosy cheeks, pale skin, and a radiant smile, Valjean realizes it would be unfair to allow her to become a nun without having experienced the outside world which a cloistered nun must renounce.

In a later chapter, Cosette eventually remembers her childhood—praying for the mother she never knew; the two ugly Thénardiers, and fetching them water somewhere "very far from Paris". In the same chapter, she asks Valjean about her mother, but he does not answer her question. When Cosette has a dream about her mother as an angel, she remarks that her mother must have been a saint. Valjean replies, "through martyrdom".